40 J. 0. Thomjison — Law of Elastic Lengthening. 



By means of the micrometer screw of catlietometer II, I 

 could measure the shifting of the zero-point accurately to 

 0*005 mm. In this direction only an extremely small inaccu- 

 racy was possible because almost without exception the gradual 

 lowering of the zero-point, in consequence of rising tempera- 

 ture and other causes, allowed the screw to be turned always 

 in the same direction. 



At the end of each series of observations the zero-point was 

 again noted by both cathetometers, and the displacement 

 measured. The difference in the results given by the two 

 instruments was on an average less than 0*005 mm. Finally, in 

 order to eliminate the influence of any change of temperature 

 in the tower, the position of the mark on the thermometer- 

 wire was again noted. 



Experiments with Brass Wire. 



As an example of the process just described I give the. fol- 

 lowing series of measurements : 



Apr. 25, lOh. 5m. Thermometer-wire, 540*71 

 Temperature, 9° '5 



Added weight. 



Time. 



Oath. I. 



Oath. II. 



Lengthening. 



kg. 



h. m. 



mm. 



mm. 



mm. 







10 8 



548-96 







0-2 



9 



41-84 



0-15 



7-12 



0-4 



11 



34-70 



a 



14-26 



0-6 



13 



27-46 



a 



21-50 



0-8 



15 



20-18 



et 



28-78 



1-0 



17 



12-81 



0-14 



36-14 



1-2 



19 



5-39 



a 



43-56 



1-4 



21 



497-885 



u 



51-065 



1-6 



23 



90-26 



0-135 



58-685 



1-8 



25 



82-60 



0-125 



66-335 



•o 



27 



548-92 



0-105 





10 h. 30 m. Thermometer-wire, 540-68 



The lowering of the zero-point during these measurements 

 was 0*04 mm. according to cathetometer I, and 0*045 mm. accord- 

 ing to cathetometer II. The thermometer-wire, which was 

 just like the main-wire, lengthened by - 03 mm. during this 

 time. The rise in temperature was accordingly T 3 W °, about 

 twice the ordinary rise. The above table indicates, as already 

 shown by the table on p. 39, that with an added weight of 1'8 

 kg. the after-effect begins to be noticeable. The lengthening 

 produced by this weight I have given, but not taken into 

 account in my later calculations. The initial load plus 1*8 kg. 

 amounted to nearly half the breaking weight. 



